Candling = black inside?

What does it mean when I candle & the inside of the egg is not visible b/c it's 80% blacked out. There is a small area that is see through, but I don't know if I should throw the egg out or if it is developing or what?
Thanks,
Kristi

I didn't think to say what day we are on...I think day 13...so not all that far along. But I noticed it on day 9-10. (I sat eggs on 3 different days & I can't honestly remember if that was from day 1 or 2 right now.)
Kristi